Waterborne traffic markings prove their worth on the road
One of the most important means of providing safe guidance to motorists on our nation’s highways is through traffic markings. All department of transportation (DOT) agencies in the U.S. and Canada use longitudinal pavement markings for center lines, lane lines and edge lines to communicate effective day and night road visibility.
Nighttime visibility is greatly enhanced with drop-on or intermixed glass beads that protrude above the surface of the traffic markings to reflect light from headlight beams back to the driver. The returned light is referred to as retroreflectivity, and retention of retroreflectivity is a key determinant of traffic-marking performance.
Line markings can reach the end of their service life either due to glass bead loss resulting in poor retroreflectivity, loss of the road base material due to chipping or abrasion, color change of the marking or loss of contrast of the marking with the asphaltic or concrete road surface. Service life of the traffic marking is often defined as the time or number of traffic passages required for a line’s retroreflectivity to decrease from its initial value to a minimum threshold value where line refurbishment or replacement is needed.
The Manual on Uniform Traffic Control Devices (MUTCD) specifies where U.S. road markings are to be provided based on the type of roadway, the width of the roadway and the average daily traffic (ADT). However, MUTCD does not specify the materials to be used in these road markings. Instead, this is left up to the state agencies who have individually developed pavement marking specifications that are either material-based, performance-based or warranty provisions. Some states use a combination of specification types.
Based on a recent National Cooperative Highway Research Program (NCHRP) survey, the total money spent on pavement markings in the U.S. and Canada exceeded $1.5 billion on nearly 4 million center-line highway miles. Traffic markings used on these roads historically have been categorized as either “durable” or “standard” (Table 1). Among the marking types classified as durables are thermoplastics, epoxy, polyurea, polyester, methyl methacrylate and preformed tape with expected service lifetimes of two to five years.
The current trend is to have contractors apply these durable markings because they require specialized equipment and highly trained workers. Included in standard traffic markings are solvent-borne paints and most waterborne paints which traditionally have expected service lifetimes of less than two years and more typically about one year. These markings are normally applied with state-owned and -operated equipment.
Standard acrylic waterborne paints have improved significantly over the past 15 years and now include high-performance styrene-acrylic latex binders for exceptional water and abrasion resistance. But what about the relative newcomer, “durable waterborne?” Are these traffic markings truly durable? Most assuredly they are, and when placed on equal footing, durable waterborne competes favorably with other marking types while offering improved economics. Before providing some compelling evidence for this, let’s examine where the traffic marking industry has been with previous generations of waterborne markings versus other traffic-marking materials.
The common dominator
Affectionately known as “traffic paints,” waterborne traffic markings have been around for many years and are still the most common striping material applied by state DOTs. According to the previously cited NCHRP survey, waterborne was used by 78% of the state agencies and was put down on 60% of the total center-line roadway mileage in the U.S. However, because waterborne is very cost effective compared with some other markings, it accounted for only 17% of the total annual expenditure for traffic markings with a typical application cost of only 5 cents per ft for a standard 4-in. (100-mm) line.
Early first-generation waterborne striping paints were based on conventional latex binders, and these paints are still often used today in less demanding applications for zone markings to delineate parking lots and garages.
Although methanol is often a component of these paints and is there to speed their dry time, conventional waterborne markings require coning or barricading to avoid tracking from vehicles prematurely driving over the lines. “No-track” dry times of 20 minutes or more are not uncommon.
Compared with these conventional waterborne paints, newer second-, third- and fourth-generation waterborne markings are applied from paints containing latex binders possessing unique fast-dry properties. These are the traffic paints that are used on our nation’s city, county, state and interstate roadways.
Modern waterborne fast-dry striping paints contain water, a latex binder, titanium dioxide, extender pigments and several additives in minor amounts. The primary functions of the latex binder are film formation, pigment binding, substrate adhesion and bead adhesion.
Currently, there are only two suppliers of fast-dry latex binders for traffic marking paints, and the paints made from these latex binders are produced by only a handful of traffic paint companies. The titanium dioxide is present to provide opacity (hiding) and the white color. Yellow striping paints are achieved with the addition of inorganic chrome yellow pigment or, more commonly, organic yellow pigments which are nontoxic. The extender pigments (predominantly calcium carbonate) are primarily used as fillers for cost reduction, but depending on the choice and amounts of extender these pigments also can serve to enhance some film properties.
Traffic paints currently used on roadways contain special latex binders that have patented “fast-dry technology” which allows the paint to dry very quickly after being sprayed onto the road surface. Vehicles can typically drive over these road markings without tracking within two minutes, which is quite remarkable considering the amount of water and other volatiles that were present in the film initially.
Like conventional striping paints, high-volume solids and the optional presence of methanol help to speed dry time in the fast-dry paints. However, the most important contributor to reducing dry time is the binder’s fast-dry mechanism. Once the traffic paint is applied, the ammonia present rapidly begins leaving the paint film. As the pH drops, the fast-dry process is activated resulting in an accelerated dry time.
On continued drying of the paint traffic marking, the latex binder particles coalesce, creating a strong durable film which must endure not only the rigors of direct sunlight and temperature extremes, but also tire abrasion and occasional snow, standing rain water and oil on the road surface.
Nobody wants to be thin
Solvent-borne paints have, for the most part, been replaced by waterborne paints and other markings in the U.S. mainly for environmental reasons. The conversion from solvent-borne began in California in 1984 with the enforcement of Rule 442 limiting volatile organic compounds (VOCs). This was followed with the EPA Clean Air Act of 1990 which mandated a limit on VOCs in traffic paint after Oct. 1, 1996, to 1.25 lb/gal or 150 g/L.
While solvent-borne traffic paints are typically formulated at 440 g/L of VOC, most waterborne traffic paints contain only 70 g/L which is well below the EPA limit. With the enactment of recent environmental legislation, there also is a current move to replace solvent-borne paints in Canada with waterborne and other low-VOC markings. Although waterborne paints have improved durability performance over the solvent-borne alkyds they replace, waterborne is often considered to be on the low end of the performance spectrum compared with durable traffic markings but with very good economics.
But what about the lower performance reputation for standard waterborne relative to some other traffic markings? Is it just perception, or is it factual, and could it be that the comparisons being made are not on an “apples-to-apples” basis?
The NCHRP Synthesis 138 report provides some predictive answers to those questions. In that report, data for a variety of traffic marking types, at a variety of film thicknesses, on several different state and NTPEP test decks were collectively examined. Based on the thermoplastic and alkyd data presented, the authors concluded that the life expectancy for similar types of traffic markings is nearly a direct function of their applied dry film thickness. That, in itself, is significant—thicker films last longer.
However, when all of the data shown for different types of traffic markings in NCHRP 138 is averaged and plotted for each film thickness with interpolation to constant ADT, the representative bar graph in Figure 1 is obtained. This graph suggests that traffic marking lifetime, or durability, is related to film thickness regardless of the traffic marking type. In other words, different types of traffic markings applied at the same film thickness would be expected to provide similar durability performance.
In considering the above finding, what is the explanation for the lower performance reputation for standard waterborne? Epoxy, polyurea and thin-line thermo are typically applied at 15-30 mils as 100% active materials. If paint is applied at a similar thickness we would expect its performance to be at least as good as these durables. Even though 15 mils is a number commonly stated for standard waterborne application, an important distinction is that waterborne is put down at 15 mils “wet” while these other markings are put down at 15-30 mils “dry.” Once the volatiles leave the waterborne paint film, the dry film thickness actually ends up being only about 9 mils, which is similar to the dry film thickness of solvent-borne alkyd paints and much less than the dry thickness of the durables. Although we would only expect a predicted lifetime of 0.5 years at that film thickness based on typical alkyd performance, standard waterborne is actually providing one-year lifetime on average with much better bead adhesion. A primary reason for this is that latex retains its elasticity much longer than alkyds, which often become brittle with aging.
Based on the prediction scenario provided above, if waterborne could be applied at 30 mils wet (18 mils dry) instead of the standard 15-mil wet line, at least a two-year lifetime performance would be expected, which would put it on a par with other durable markings at that film thickness. Figure 2 shows the wet and dry film thickness comparisons of the standard fast-dry paint, durable fast-dry paint and a typical epoxy.
Although they are excellent performers in standard 15-mil wet film thickness markings, competitive second- and third-generation fast-dry latex binders are limited in their ability to form films without cracking when applied at higher film thickness.
To advance waterborne to durable status, the two producers of waterborne fast-dry traffic binders have developed fourth-generation latexes that have the capability to be formulated into traffic paints that can be applied at up to 30 mils wet with good film formation and fast dry times.
Most of the traffic paint currently used throughout the country is standard waterborne; however, many states are now either currently using or are seriously considering durable waterborne and the establishment of durable waterborne specifications.
Although the two latex producers have taken different approaches in the design of the high-performance durable latex binders, both products have documented long-term performance in durable traffic paints on state and National Transportation Product Evaluation Program (NTPEP) “test decks” and also in commercial practice in several states.
The line is 75
The NTPEP was established in 1994 as a pooled-fund engineering and technical service program operated by the American Association of State Highway & Transportation Officials (AASHTO). Some state agencies and most traffic paint companies use NTPEP test decks as a means to evaluate and screen commercial and experimental paints for possible bid in participating states.
Transverse paint lines from competitors are put down side-by-side on these decks, and after sufficient dry time previously diverted traffic is allowed to flow over the lines. Compared with the normal longitudinal lines on roadways, the transverse lines get crossed by vehicles much more frequently providing accelerated test data. Upon application of the markings, the test decks are periodically monitored for two years or more, or until line failure occurs. For a fee, the field performance is provided to those submitting traffic marking samples for evaluation.
The northern NTPEP test deck site for year 2000 was on a section of I-80 north of Harrisburg, Pa. This PennDOT site traditionally has been one of the most widely used and referenced for NTPEP field evaluations. Under carefully controlled conditions, six standard second- and third-generation fast-dry waterborne paints along with two fourth-generation durable fast-dry waterborne paints were applied to both concrete and asphalt sections of the deck. Both sets of paints were provided by a leading traffic paint company in the U.S., and each contained competitive binders from the two latex binder producers. The standard waterborne paint was applied at 15 mils wet (9 mils dry) using small standard Type II drop-on glass beads. The durable waterborne was applied at 30 mils wet (18 mils dry) with large beads. After sufficient dry time, the traffic was diverted back over the lines and the stripes were periodically monitored over the course of three years.
Representative performance results are shown in Figure 3 for retroreflectivity retention in the skip line area (area between the wheel tracks), and in Figure 4 for durability in the wheel track area. A durability rating of 10 indicates that 100% of the line remains, and this is the starting value for each line when applied in July 2000.
The information presented in the graphs are the combined average data for the six standard paints and for the two durable paints, respectively. From these graphs the exceptional performance of the durable waterborne is revealed, and although not shown here, similar performance trends for retroreflectivity on asphalt and for durability on concrete were obtained.
About 70% of the initial retroreflectivity and about 75% of the paint lines still remained after three years, and retroreflectivity was still high for good nighttime visibility. In contrast, the standard paint lines lost 50% of their initial reflectivity in just two years with about 40% of the line remaining. That is still relatively good performance for standard 15-mil wet paints, but it pales by comparison to the durable waterborne marking’s performance. Results for a similar data set were obtained for yellow lines though retroreflectivity and durability was less than for white lines, which is considered normal.
Durable is solid choice
Selection of pavement marking materials for highway delineation is generally made on the basis of retroreflectivity, durability and cost. One DOT and a traffic paint company have claimed that in their climate extremes of Southwest U.S. and Canada, respectively, durable waterborne is providing a three- to five-year lifetime with application at 20-30 mils wet. That is exceptional performance which is at least on par with other durables, and the surprising performance claims are supported by the PennDOT NTPEP data.
As stated earlier, even though waterborne paint is on the majority of the nation’s roadways, it represents less than 20% of the total cost for traffic markings. Thermoplastic is the next largest striping material used, and by comparison it is striped on only 23% of the roads but with 35% of the total cost. Cost extremes for durable traffic markings are rather dramatic. Durable waterborne is the most economical at about $0.10 per ft, while preformed tape is at the other extreme of $2.50 per ft or higher.
One of the reasons for the higher cost of other durable markings is that the binders used are often more expensive than latex on a dry solids basis. An even more significant factor in their cost is that they are typically applied at higher, and sometimes much higher, film thickness which is an important factor in their extended durability.
The application equipment for durable binders also tends to be more costly than for waterborne, and surface preparation or line removal prior to application frequently is required which significantly adds to their application cost.
An important advantage for waterborne is that it can be applied over dry or damp pavement and also over almost any previous pavement-marking material. When all factors are considered, standard waterborne with annual application is often a better value than many durable markings, and when applied at appropriate film thickness, durable waterborne offers even greater value due to extended line marking lifetime, lower overall cost, improved retroreflectivity with large or mixed beads and high bead retention.